1. Field of the Invention
The present invention relates to an optical unit for use in a laser beam printer, and more particularly to an optical unit which guides a laser beam from a laser diode to an object to be scanned, through a group of focusing lenses and a scanner.
2. Description of the Related Art
In general, an optical unit incorporated in a laser beam printer or the like is designed such that a laser beam output from a laser diode is guided first to a scanner and then to a photosensitive body, i.e., an object to be scanned. The photosensitive body is scanned with the laser beam at a constant speed. When guided from the laser diode to the photosensitive body, the laser beam passes through a group of focusing lenses. By these focusing lenses, the laser beam is made to have a cross sectional shape having desirable characteristic, and then fall on the predetermined position on the surface of the photosensitive body.
This type of optical unit is made up of first and second optical systems which are isolated from each other. The first optical system converges the laser beam output from the laser diode, while the second optical system focuses the laser beam on photosensitive body. The scanner is arranged on between the first and second optical systems, as will be detailed later.
The first optical system is a combination of lenses, such as an aspheric glass lens, plastic lenses, etc. The second optical system is a combination of lenses, such as an f.theta. lens, etc. By this f.theta. lens, the deflection angle at which a laser beam is deflected by the scanner is changed in proportion to the position at which the laser beam is focused on the photosensitive body and which is expressed in relation to the main scanning direction.
The optical unit can be classified as either a pre-object type or a post-object type, depending upon the arrangement among the first optical system, the second optical system, and the scanner. If only the second optical system is used for focusing, the optical unit is classified as a pre-object type. If the scanner is interposed after the focusing optical systems, the optical unit is classified as a post-object type.
U.S. Pat. No. 4,627,685 discloses a post-object type optical unit, wherein a scanner is constituted by a polygonal mirror whose mirror faces are part of convex cylinders. In the optical unit disclosed in the U.S. patent, the field curve of the laser beam or image focused on the photosensitive body can be reduced. However, where convex cylindrical mirrors are employed, as in this U.S. patent, the f.theta. characteristic remains uncorrected. In addition, a variation in the cross sectional size of a laser beam used for scanning a photosensitive body also remain uncorrected.